System for access to a synchronous network of the type comprising transmitting equipment and receiving equipment

ABSTRACT

A synchronous frame is formed by inserting data in virtual containers that are inserted in the fame. The containers are concatenated with each other. Each concatenated container indicates the total number of containers, and the order number of the particular container. A receiver responsive to the frame delivers to its output only the containers with the indications in the order of the order numbers.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based on, and claims priority from, FrenchApplication Number 99/04852, filed Apr. 14, 1999, the disclosure ofwhich is hereby incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention concerns a system for access to a synchronousnetwork of the type comprising transmitting equipment and receivingequipment.

The present invention applies to any type of transportation bysynchronous multiplexing, the two main currently known types of whichare synchronous digital hierarchy (SDH) and the American hierarchy knownas SONET (Synchronous Optical NETwork).

Although the invention is described in relation to the first of thesehierarchies, it will be understood that it can apply to the second, thefundamental principles of which are equivalent if not identical.

BRIEF SUMMARY OF THE INVENTION

A statement is given below of the characteristics defined in the ITU-TRecommendation G.707 relating to synchronous digital hierarchy (SDH)which are necessary for an understanding of the present description.

This Recommendation G.707 defines a basic 155.520 Mbits/sec frame knownas STM-1 (Synchronous Transport Module level 1). It also definescontainers in each of which the data to be transmitted are included andan overhead known as POH (Path Overhead) in order to form what is calleda virtual container VC. Different types of virtual containers areprovided according to their capacity and therefore the rate which theymake it possible to obtain. Thus virtual container VC4 allows a rate ofapproximately 150 Mbits/sec, virtual container VC3 a rate ofapproximately 48 Mbits/sec, VC2 a rate of approximately 7 Mbits/sec andVC12 a rate of approximately 2 Mbits/sec.

In FIG. 1, four successive basic STM-1 frames can be seen, formingbetween them what is referred to in the remainder of the presentdescription as a multiframe. Each basic frame, with a duration of 125μs, is organised in 9 lines of 270 octets. It is composed of a sectionoverhead SOH, a pointer PTR and a useful load.

The useful load consists of a virtual container VC4 whose position inthe STM-1 frame is defined by the pointer PTR. The value of this pointerPTR can be modified when the frame rate is shifted by plus or minus withrespect to the rate of the virtual container VC4. Positive or negativejustification mechanisms are also provided in order to take account alsoof these frequency shifts.

In FIG. 2, a virtual container VC can be seen, which can be of the VC12or VC2 type and which consists, distributed over four basic STM-1frames, of four parts of identical size, each having a header octet(reference in the standard V5, J2, N2 or K4) and a load available to theuser.

In the remainder of the description, each of these parts will be named avirtual subcontainer equivalent to a quarter of a virtual container,which will therefore be either a virtual subcontainer of VC12 or avirtual subcontainer of VC2.

The size of a virtual subcontainer is either 35 octets if it is asubcontainer of VC12, or 107 octets if it is a subcontainer of VC2. Eachvirtual subcontainer has a header octet (V5, J2, N2 or K4) and therefore34 or 106 useful load octets. The set of header octets constitutes thepath overhead POH of the corresponding virtual container.

The respective functions of these overhead octets are described inRecommendation G707. It will merely be noted that octet J2 normallyserves to periodically transmit an identifier for an access point to apath of lower level. This path access point identifier is defined by amessage in 16 octets.

It should be noted that a multiframe may contain up to 63 VC12 virtualcontainers or up to 21 VC2 virtual containers.

Each VC12 or VC2 virtual container is inserted in a TU12 or TU2tributary unit. Because the data of the VC12 or VC2 virtual containerand the TU12 or TU2 tributary unit are not synchronous, the VC12 or VC2virtual container can float within this tributary unit so that a pointeris necessary in order to indicate its start in the tributary unit TU.This pointer is transported by the octets V1 and V2 situated in theoverhead of the tributary unit TU and points to the first octet of thecorresponding virtual container, that is to say the octet V5 of theoverhead POH, as can be seen in FIG. 1 a. It should be noted that thispointer is defined for a multiframe and that it can be modified when thetransmission rate of the tributary unit is shifted by plus or minus withrespect to the rate of the VC2 or VC12 virtual container. Positive ornegative justification mechanisms are also provided to take account alsoof these frequency shifts.

Finally, each tributary unit TU is inserted in four VC4 virtualcontainers of four successive STM-1 frames. Each tributary unit TU issynchronous with the VC4 virtual container containing it.

At the present time, an SDH network can offer at a maximum only fiveuseful transportation rates, which correspond to the following virtualcontainers used:

virtual container of the VC4 type: 150 Mbits/sec,

virtual container of VC3 type: 48 Mbits/sec,

virtual container of the VC2 type: 7 Mbits/sec,

virtual container of the VC12 type: 2 Mbits/sec,

virtual container of the VC11 type: 1.5 Mbits/sec.

The consequence of the transportation of flows of intermediate rates isnot only poor filling of the virtual containers and therefore of the SDHsynchronous paths but also, because of the SDH multiplexing structure, aloss of resources in the form of VC11, VC12, VC2 or VC3 virtualcontainers which could be available per STM-1 frame for other flowsadapted to these virtual containers.

Thus not only are the rates, for example between 50 and 100 Mbits/sec,transported by a VC4 virtual container with a low filling rate ofbetween 33% and 66%, but also it is impossible to use, for other flows,the virtual containers VC of lower order (VC11, VC12, VC2 or VC3) madeunavailable in the STM-1 frame which is occupied entirely by this poorlyfilled VC4 virtual container.

To resolve this problem, Recommendation G.707 provides for the virtualconcatenation of TU-2 units in a VC-4 container of higher level. Thisconcatenation allows the transportation of data in m×TU-2 units withoutusing the concatenation indication in the pointer octets.

It is also mentioned that the processing of the pointer in intermediateequipment may cause differences in the delay of the signals of virtuallyconcatenated individual VC2 containers.

There is also known, in order to partly resolve this problem of fillingSDH containers, according to the patent document EP-A-814 580, a hybridmultiplexer which comprises first reception means for receiving thecontent of a large-capacity SDH container, concentration means forreducing the number of ATM cells in the container received byeliminating the empty cells which were contained therein, generationmeans for generating a lower-capacity SDH container with the ATM cellsretained and any empty padding cells, multiplexing means for effectingthe SDH synchronous multiplexing of the lower-capacity containers andtransmission means for transmitting the SDH container thus generated.

In the present invention, the concatenation in question is alwaysvirtual. The latter term will not be used routinely to designate it, butit will be understood that it will nevertheless be implied.

However, no mechanism is provided by Recommendation G.707 for recoveringthe concatenated virtual containers if these are subject, during theirtransportation in the SDH network, either to a change in position in theframe, or movements of their TU2 or TU12 unit pointers.

Likewise, no mechanism is provided for putting the concatenatedcontainers back in phase when these are subject to large phase shiftsgreater than one multiframe.

In addition, no mechanism is provided by Recommendation G.707 forprojecting the service rate into containers and then concatenating them,in order to adjust the service rate to a reserved rate.

The purpose of the present invention is to propose a system for accessto a synchronous network which makes it possible to resolve theseproblems, and in particular to propose such a system for transmittingdata over a synchronous network of the SDH type so as best to use thecapacity of the frames both for the source data flow and for the otherflows of the intermediate equipment of the SDH network.

Such a system for the transmission of data by a synchronous network isof the type which comprises at least one item of transmitting equipmentand at least one item of receiving equipment, the said data beingsupplied at the input of the said transmitting equipment in order to beinserted in concatenated virtual containers which are themselvesinserted in synchronous frames in order to be transmitted to the saidreceiving equipment.

According to one characteristic of the present invention, the saidtransmitting equipment has a unit for measuring the rate of the incomingdata flow and a parameterising unit which deduces, from the measurementmade by the said measuring unit, the total number of virtual containersof lower order to be virtually concatenated in order to transport thesaid data flow in the said frames, the header of each concatenatedcontainer carrying a message giving the total number of concatenatedcontainers and the order number of the said container amongst the saidconcatenated containers, the said receiving equipment delivering onlythe containers carrying the said message, and this in the order given bythe said message.

According to another characteristic of the invention, the said networkis of the SDH type in accordance with Recommendation G.707 and, for agiven concatenated virtual container, the said message consists of thesixteen J2 header octets successfully transmitted in sixteen successivemultiframes.

According to another characteristic of the invention, the said receivingequipment functions according to at least two distinct phases, alearning phase in which, by means of the message carried by each of thesaid containers, the said receiving equipment marks, in the framesreceived, the position of each concatenated container received, and anormal functioning phase in which, on the basis of each position thusdetermined, it delivers the said concatenated containers to a unit forrecovering the said data.

According to another characteristic of the invention, the saidtransmitting equipment functions according to at least two distinctphases, an initialisation phase during which the measuring unit measuresthe incoming flow rate and the parameterising unit derives therefrom thetotal number of virtual containers to be reserved for the transportationof the said data of the incoming flow and then a normal functioningphase during which the said data are inserted in the said reservedvirtual containers, the said transmitting equipment leaving the saidnormal functioning phase in order to enter the said initialisation phasewhen the said measuring unit indicates that the said measured rate isgreater than the maximum rate which can be offered by the total numberof reserved concatenated containers.

According to another characteristic of the invention, the saidtransmitting equipment comprises a container formation unit in order toform the said concatenated containers by means on the one hand of theuseful data of the incoming flow and on the other hand, if these are notavailable in sufficient quantity at the time in question, by means ofpadding data necessary for saturating the rate offered by the saidvirtual containers.

According to another characteristic of the invention, in a firstembodiment, each subcontainer has, in addition to its header, a lengthoctet which represents the quantity of useful data and/or the quantityof padding in its useful load.

According to another characteristic of the invention, and in anotherembodiment, the said transmitting equipment has a buffer which issupplied by the said incoming flow and which is designed to deliver, atits request, blocks of data useful to the said formation unit, and acontrol unit which either controls the delivery by the said buffer ofone or more blocks of useful data when these are available in the saidbuffer memory, or delivers blocks of padding data when the quantity ofuseful data in the said buffer is less than the quantity of data in ablock. Each of the said blocks of data has for example the size of avirtual subcontainer so as to fill its useful load, each virtualsubcontainer being either of a type where it contains useful data or ofa type where it contains padding data.

According to another characteristic of the invention, each subcontainercomprises, in addition to its header, an octet representing the type ofdata which it contains.

According to another characteristic of the invention, the said typeoctet has seven bits which express the word 1010101 when the saidsubcontainer is transporting useful data and the binary word 0101010when it is transporting padding data, the said octet being recognised atthe receiving equipment by calculating the Hamming distance between theseven bits of the octet received and the value 1010101 and by comparingthis distance with the figure four, the subcontainer received being ofthe type where it contains useful data if the said distance is less thanfour and being of the type where it contains padding data if thisdistance is greater than or equal to four.

According to another characteristic of the invention, the said bufferdelivers the respective values of a high pointer and of a low pointer,the said control unit delivering blocks of padding data when thedifference between these two values is less than the size of asubcontainer, until the said difference once again becomes greater thanthe size of a subcontainer, the said control unit then controlling thedelivery, by the buffer, of the data blocks which it contains.

According to another characteristic of the invention, the said incomingflow is a flow of ATM cells and each of the said blocks of the data hasthe size of one or more ATM cells.

According to another characteristic of the invention, the said blocks ofdata are stored in a multiframe, consecutively filling the reservedcontainers.

According to another characteristic of the invention, the said blocks ofdata are stored in a multiframe distributing the said blocks one afterthe other on the reserved containers.

According to another characteristic of the invention, the said receivingequipment is provided at its output with a buffer which is supplied bythe recovered data extracted from the concatenated virtual containersreceived, at the extraction rhythm of the said data, and which is readat a regular rhythm.

According to another characteristic of the invention, the said messagealso carries an identifier for the said transmitting equipment.

The present invention also concerns a method of transmitting data bymeans of a synchronous network of the type in which the said data areinserted in virtual containers which are themselves inserted insynchronous frames in order to be transmitted.

According to one characteristic of the present invention, the saidtransmission method consists of measuring the rate of the incoming dataflow, deriving from the result of the said measurement the total numberof virtual containers to be concatenated in order to transport the saiddata flow in the said frames, and providing each of the said containersto be concatenated with a message giving the total number ofconcatenated containers and the order number of the said containeramongst the said concatenated containers.

According to another characteristic of the invention, the said networkis of the SDH type in accordance with Recommendation G.707 and the saidmethod is characterised in that, for a given concatenated virtualcontainer, the said message consists of sixteen J2 header octetssuccessfully transmitted in sixteen successive multiframes.

According to another characteristic of the invention, on reception of aframe containing concatenated containers, two distinct steps areimplemented, a learning step in which, by means of the message carriedby each of the said containers, the position of each concatenatedcontainer received is recovered in the received frames and a normalfunctioning step in which, on the basis of each position thusdetermined, the said concatenated containers are delivered so that thesaid data can be recovered.

According to another characteristic of the invention, it consists ofputting the reserve containers back in phase, on reception, by analysingthe offset in the octets constituting the message carried by each ofthem.

According to another characteristic of the invention, on transmission,two distinct phases are implemented: an initialisation phase duringwhich a measurement of the rate of the incoming flow is carried out andthe total number of virtual containers to be reserved for thetransportation of the said data of the incoming flow is derived, and anormal functioning phase during which the said data of the incoming floware inserted in the said reserved virtual containers, the said normalfunctioning phase being abandoned for the said initialisation phase whenthe said measured rate is greater than the maximum rate which can beoffered by the total number of reserved concatenated containers.

According to another characteristic of the invention, it consists ofusing, in order to form the said concatenated containers, on the onehand useful data of the incoming flow and on the other hand, if theseare not available in sufficient quantity at the time in question,padding data necessary for saturating the rate offered by the saidvirtual containers.

According to another characteristic of the invention, the said messagealso carries an identifier for the said transmitting equipment.

The present invention also concerns a synchronous frame for thetransportation of data flows, the said data being inserted in virtualcontainers which are themselves inserted in the said synchronous frame.

According to another characteristic of the invention, each of the saidcontainers which are concatenated with each other carries a messagegiving the total number of concatenated containers and the order numberof the said container amongst the said concatenated containers.

According to another characteristic of the invention, the said frame isof the SDH type in accordance with Recommendation G.707 and is alsocharacterised in that, for a given concatenated virtual container, thesaid message consists of sixteen J2 header octets successfullytransmitted in sixteen successive multiframes.

According to another characteristic of the invention, each subcontainerhas, in addition to its header, an octet of the type which indicateswhether it is carrying, in its useful load, useful data or padding data.

According to another characteristic of the invention, the said typeoctet has seven bits which express the word 1010101 when the saidsubcontainer is transporting useful data and the binary word 0101010when it is transporting padding data.

According to another characteristic of the invention, each subcontainerhas, in addition to its header, a length octet which represents thequantity of useful data and/or the quantity of padding in its usefulload.

BRIEF DESCRIPTION OF THE DRAWINGS

The characteristics of the invention mentioned above, as well as others,will emerge more clearly from a reading of the following description ofan example embodiment, the said description being given in relation tothe accompanying drawings, amongst which:

FIG. 1 is a diagram showing the structure of the frames in a synchronousnetwork of the SDH type to which the present invention can be applied,

FIG. 2 is a diagram showing the structure of the virtual containerswhich are concatenated according to the present invention,

FIG. 3 is a block diagram of a system for the transmission of data by asynchronous network according to the present invention,

FIGS. 4 a and 4 b are a diagram illustrating the filling of theconcatenated virtual containers according to two distinct modes,

FIG. 5 is a diagram illustrating the structure of the message carried byeach of the concatenated virtual containers, and

FIGS. 6 a to 6 c are diagrams illustrating the filling of the virtualcontainers by means of the useful data and the padding data according totwo embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

There will be considered; in relation to FIG. 3, transmitting equipment10 in communication with a receiver 20 through a synchronous network 30,for example of the SDH (Synchronous Digital Hierarchy) type based onRecommendation G.707 of the ITU-T referred to above or of the SONETtype.

The transmitting equipment 10 has a service input 11 through which thedata to be transmitted are input. In the context of the presentinvention, these data can be in various formats such as for example: ATMcells, broad-band digital signals of the NRZ binary type, data to theG.703 PDH format, audio-digital signals to the AES/UER format orcompressed audio-visual signals to the MPEG2-TS format, IEEE 802.3 orEthernet data frame, etc.

These data are supplied to a buffer 12 of the type known in the art asFIFO (First In, First Out). This buffer 12 delivers the useful data DUwhich it has stored to a container formation unit 13, at the request ofthe said unit 13. A parameterising unit 14 is controlled, as explainedbelow, by a unit 15 for measuring the data flow rate at the input 11 ofthe transmitting equipment.

The buffer 12 also delivers the respective values of a high pointer phand a low pointer pb representing its filling level. These pointervalues are supplied to a rate control unit 16 which delivers, to thecontainer formation unit 13, as will be explained below as a function ofthe filling levels of the buffer 12, padding data B.

The function of the unit 13 is to form, by means of the useful data DUdelivered at its own request by the buffer 12 and padding data Bsupplied by the rate control unit 16, C2 or C12 containers. It receives,from the parameterising unit 14, various parameters and in particularthe number of containers N to be concatenated and the type, C2 or C12,to be considered.

The unit 13 delivers the C2 or C12 containers which it forms to avirtual container formation unit 17 provided for adding V5, J2, N2 andK4 overhead octets to the C2 or C12 containers and thus form VC2 or VC12virtual containers. The unit 17 receives, from the parameterising unit14, a J2 octet which constitutes one of the said overhead octets.

The virtual containers thus formed are delivered to a transmission unit18 which, as explained above, inserts them in VC4 containers, whichprojects the said VC4 containers into STM-1 frames and transmits thesaid STM-1 frames, via the SDH network 30, to the receiver 20.

It will be recalled that the VC2 and VC12 containers are defined on amultiframe defined as being the succession of four STM-1 frames whilstthe VC4 virtual container for its part is defined on an STM-1 frame.

The receiver 20 receives the said frames through a reception unit 21which, in a manner known per se, after interpretation of the pointersPTR (which are not necessarily the same as those which were determinedby the transmitting equipment 10 notably because of the passage of thesaid frames through the SDH network 30), recovers the VC4 containers,and then the TU2 or TU12 tributary units which the said containerscontain and, after interpretation of the corresponding pointers V1 andV2, delivers VC2 or VC12 virtual containers to a container recovery unit22 provided for recovering the corresponding C2 or C12 containers. Theunit 22 is controlled by an overhead analysis unit 23 which analyses theoverhead octets V5, J2, N2 and K4 and enables it to extract only the C2or C12 containers resulting from the concatenation, as will be explainedbelow. Finally, the C2 or C12 containers supplied by the unit 22 aredelivered to a data recovery unit 24 which on the one hand recovers theinitial data and on the other hand recovers the rhythm and makes regularthe rate of the said data which are then delivered at a service output25.

The unit 24 consists for example of a buffer of the FIFO type, where thewriting of the data is synchronous with the C2 or C12 containersdelivered by the unit 22 and is therefore because of this sporadic, andwhere the reading of the data is on the other hand slaved to its fillinglevel making the data output rate regular.

A description will now be given of the functioning of the transmittingequipment 10. It functions in two distinct phases: an initialisationphase and a normal functioning phase.

The initialisation phase is implemented when the transmitting equipment10 is started up. There, the different cards constituting the equipmentare configured and any anomalies and alarms which may prevent start-upare detected.

During this phase, the rate measuring unit 15 measures the rate of theincoming data flow and transmits the result of this measurement to theparameterising unit 14, which then derives therefrom the number N ofVC12 or VC2 virtual containers which it is necessary to reserve in orderto transmit the service data issuing from the input 11 and which storesthis value N. For example, the number N of VC12 or VC2 containers to bereserved is equal to the integer just greater than the ratio of the ratethus measured D_(mes) to the useful data rate D_(vc12) or D_(VC2)corresponding to a VC12 or VC2 container, taking into account thedifferent overheads necessary.

For example, if the measured rate D_(mes) is 25 Mbits/sec and thevirtual containers in question are VC2 containers whose rate D_(vc2) is6784 kbits/sec, the number N will be equal to 4.

It will be understood that, in the context of Recommendation G.707referred to above, the number N will be no more than 63 for VC12containers offering a useful data rate D_(vc12) and 21 for VC2containers offering a useful data rate D_(vc2). The exact values of therates D_(VC12) and D_(VC2) will be seen subsequently.

If the number of containers to be reserved is equal to the number Nalready used during a previous operating phase (the number N is storedin the parameterising unit 14), no modification is necessary and theequipment goes directly into the subsequent operating phase.

It should be noted that the fact that the number N is stored makes itpossible to restart the equipment in its last state, without humanintervention, after a mains power cut has occurred.

If the number of containers to be reserved is greater than thepreviously stored number N, its new value N is stored in place of theprevious one in order to be taken into account. A confirmation from theuser may be requested in order to make it possible to artificiallyincrease, during transmission, the number of concatenated VC12 or VC2virtual containers in expectation of an increase in the input rate.

The number N and the type of C2 or C12 containers used are stored by theunit 14 in order to be delivered, during the subsequent operating phase,to the container formation unit 13.

During this phase, the transmitting equipment 10 functions as follows.

At each moment, the measuring unit 15 measures the rate D_(mes) of thedata flow at the input 11. The parameterising unit 14 checks whether thedata rate D_(mes) thus measured can be transported by the N VC12 or VC2containers reserved during the initialisation phase.

If such is not the case, an alarm is activated and the transmittingequipment exits from the operating phase in order to enter a newinitialisation phase where a new number N of VC12 or VC2 virtualcontainers to be reserved will then be calculated.

It should be noted that the operator can at any time interrupt thetransmission. The equipment then exits from the operating phase in orderlater to resume an initialisation phase.

The container formation unit 13, on the basis of the values of theparameters which are transmitted to it by the parameterising unit 14,notably the value N and the type of VC12 or VC2 virtual containers used,requests the buffer 12 to supply useful data DU to it in the form of oneor more blocks of octets or, if these are not available in the requiredquantity, to supply to it, via the rate control unit 16, padding data Bwhich will then serve to saturate the rate offered by the N containersreserved.

It should be noted that the size of the blocks of octets depends on theformat of the data to be inserted in the container. For example,according to the invention, if these are binary data of the NRZ type, ablock will correspond to the octets which are necessary for filling theuseful load of a S-VC12 or S-VC2 subcontainer, that is to sayrespectively 33 or 105 octets. If they are data in the form of ATMcells, a block will correspond to an ATM cell, that is to say to 53octets.

The unit 13 then forms C12 or C2 containers from these useful data DUand padding data B.

The blocks of data can be stored in a multiframe either by filling the Ncontainers consecutively (see FIG. 4 a) or by distributing the blocks ofdata one after the other over the N reserved containers (see FIG. 4 b).

In the first case (FIG. 4 a), the blocks are inserted container aftercontainer so that it is necessary to await the complete filling of the Ncontainers before being able to transmit the multiframe. The latency ofthe process is then four frames, that is to say, for a frame duration of125 μs, a latency of 500 μs.

In the second case (FIG. 4 b), the filling of the N reserved containerstakes place frame by frame. Each block of data is distributed over theoctets of the N reserved containers. A frame can be transmitted as soonas it is filled, so that the latency of the process is only one frame,that is to say, for a frame duration of 125 μs, a period of 125 μs.

The unit 17 attaches to each subcontainer a header octet, which iseither the octet V5, or the octet J2, or the octet N2, or the octet K4in accordance with Recommendation G.707 and thus forms the VC2 or VC12virtual containers according to circumstances.

The unit 17 continuously numbers each container used and thus positionsits J2 octet so as to form a so-called J2 identifier message,constructed from a certain number of J2 octets present in the pathoverhead POH of the same number of successive multiframes. The number ofJ2 octets forming the said J2 identifier is advantageously 16 in orderto be in accordance with Recommendation G.707, which provides for theformation of such a message without however defining the contentthereof. Likewise, in more general terms, the format of the J2identifier is in accordance with Recommendation G.707.

According to the present invention, the J2 identifier forms a messagespecific to the equipment used in order to be able to identify withreliability the concatenated containers VC belonging to a given item ofequipment amongst all the virtual containers received in the multiframe.It also makes it possible to verify the sequencing of the concatenatedcontainers and the integrity of the concatenation.

This makes it possible to resolve the problems of marking and extractionof the concatenated virtual containers received, problems related to thefact that, in the SDH network 30, the values of the pointers V1 and V2allocated to these virtual containers are not necessarily preservedbecause of their change in position. In addition, the order of thecontainers may also not be preserved and their position within the framemay be modified in the network between transmitting equipment andreceiving equipment, for example cross-connection, multiplexing etcequipment of the SDH network.

As can be seen in FIG. 5, the message carried by the J2 octets consistsessentially of three fields: an identifier for the IEE transmittingequipment, the order number n of the container concerned and the totalnumber N of containers which have been concatenated by the transmittingequipment identified.

These fields are for example expressed in clear in so-called ASCII code,the first in six octets, the second in two octets and the third also intwo octets. A separator may be provided between the order number n andthe total number N of containers reserved by multiframes.

In addition, one octet, for example the first, of the J2 identifier may,in accordance with Recommendation G.707, contain a cyclic redundancycheck (CRC) code associated with the J2 identifier previously received,and the other octets are intended for the message proper.

The J2 identifier also contains octets reserved for other applications(here four in number).

The VC12 or VC2 virtual containers formed by the unit 17 are transmittedto the unit 18, which projects them into the TU12 or TU2 units, whichare inserted in the virtual containers VC4 of successive frames in orderthen to be transported by the SDH network 30 in multiframes formed byfour STM-1 frames.

As for the receiving equipment 20, this functions as follows. The STM-1frames received from the SDH network 30 are analysed in the unit 21,which then delivers, from corresponding containers VC4, virtualcontainers VC12 or VC2. In order to take account of the differences inthe values of the pointers between concatenated virtual containers dueto the change in their position when they are transmitted in the SDHnetwork, it may be necessary to store at least four consecutive STM-1frames, or even several multiframes.

This may in particular be the case when two concatenated containersundergo, in the SDH network, a significant phase shift exceeding onemultiframe. The J2 identifier will make it possible to put them back inphase as long as this phase shift does not exceed eight multiframes.This is because, if two concatenated containers are shifted in phase byP multiframes, at a given moment, one will deliver the K^(th) J2 octetof the J2 message and the other will deliver the (K+P)^(th) J2 octet. Itwill be understood that, during the analysis of the J2 octets received(which is carried out during the learning phase of the receiver 20 (seebelow)), it is possible to deduce the phase shift P between these twocontainers.

Since the J2 octet comprises 16 octets, the maximum phase shift whichcan thus be determined is 8 multiframes. For example, a phase shift ofP(P>8) multiframes can be interpreted as a positive phase shift of Pmultiframes or as a negative phase shift of (16-P) multiframes.

It will be understood that it may then be necessary to store up to eightmultiframes in order to recover these phase shifts.

The unit 22 analyses the headers of these VC2 and VC12 virtualcontainers and, in particular, extracts the J2 octet, which it thensupplies to the unit 23. The unit 22, as disclosed below, then deliversthe C2 or C12 virtual containers which had been concatenated with a unit24 which disassembles them so as to recover the flow of initial data atthe output 25.

More precisely, the receiving equipment 20 functions according toessentially three distinct phases, the initialisation phase, thelearning phase and the operating phase.

The initialisation phase is implemented when the reception equipment isstarted up. The different cards which constitute the equipment areconfigured and any anomalies and alarms which may prevent correctfunctioning of the system are detected. Once this initialisation phasehas been carried out, the equipment goes into the learning phase.

During the learning phase, the J2 octets delivered by the unit 22 areread by the unit 23, which reconstitutes and interprets the J2identifier carried by the J2 overhead octets (for example sixteen innumber) of successive multiframes. The unit 23 then delivers the valueof the number N of containers which have been concatenated at thetransmitting equipment in question as well as the value of the ordernumber n of the container in question. These two values are recorded bythe unit 23 and delivered to the unit 22, which will thus be able toenter its third operating phase. At the end of this phase, the receivingequipment 20 is synchronised with the transmitting equipment 10.

It is during this learning period that the phase difference between theconcatenated containers is determined.

It should be noted that the containers which do not relate to thetransmitting equipment in question are rejected without interpretationof their J2 octet.

In addition, it should be noted that, during this learning phase, therecovery of the data flow is deactivated.

When it possesses the position in the multiframe of the N containersused, the receiving equipment passes to the operating phase.

During the operating phase, there is first of all a reception by theunit 21 of the receiving equipment 20 of the frames issuing from thetransmission path 30.

The processings related to the SDH transportation layer between thephysical interface and the S2 or S12 layer are described in thefunctional model of Recommendation G.707 mentioned above. At eachmultiframe received, the pointers of the containers situated in the V1and V2 overhead of the TU12 or TU2 tributary units are saved in amemory. There is then extraction of the VC12 containers or VC2containers from the TU12 or TU2 tributary units.

The VC12 or VC2 virtual containers received are in accordance withRecommendation G.707. The unit 23 processes the octets of their overheadV5, J2, N2 and K4 and reconstitutes the message transported by the J2identifier. The virtual containers which have been received but whichhave not been recognised as appearing amongst those which are pointed toby the identifier J2 determined during the synchronisation phase arerejected. For the others, the blocks of useful data contained in the C12or C2 containers are extracted by the unit 22 from the VC12 or VC2virtual containers delivered by the unit 21 and are then delivered tothe unit 24, which then delivers the flow of data recovered at theservice output 25.

The message in 16 octets transported by the J2 identifier is interpretedby the unit 23 even outside the learning phase. It makes it possible toverify that the configuration of the concatenated containers in themultiframe has not changed. Should it happen to vary, the equipmentshould go back into the learning phase.

When the service data transmission rate at the input 11 is less than therate reserved by means of the N VC2 or VC12 containers, it is necessary,in order to saturate the path rate, to add to the useful data DU dataknown as “padding” data B, that is to say ones which do not representuseful information.

The introduction of the padding data B into a C2 or C12 container maytake place either directly in the container itself or at the ATM layerconsidering the padding cells which are then introduced into thecontainer just like any other ATM cell.

With regard to direct introduction, the padding data B may betransmitted mixed at the octet level with useful data DU within one andthe same virtual subcontainer. The configuration of a virtualsubcontainer is then the one depicted in FIG. 6 a. This virtualsubcontainer has a useful load CU in which the useful data DU areinserted and a padding B in which the padding data are inserted. It alsohas a header octet H (which is one of the octets V5, J2, N2 or K4)followed by an octet L which indicates the length, for example expressedin numbers of octets, of the useful load CU.

It will be recalled that a virtual subcontainer comprises 35 or 107octets. Consequently, having regard to the header octet H of the octetL, the useful load CU will have a maximum length of 33 or 105 octets.The maximum rate D_(VC2) provided by a VC2 virtual container (four S-VC2virtual subcontainers) will then be per multiframe 6720 kbits/sec andthe rate D_(vc12) for a VC12 virtual container (four S-VC12 virtualsubcontainers) 2112 kbits/sec.

Still with regard to direct introduction, the padding data can also betransmitted by being collected together in specific subcontainers, whichare then mixed with subcontainers of useful data.

FIG. 6 b depicts a subcontainer of useful data whilst FIG. 6 c depicts apadding subcontainer. It will be noted that each contains an indicationof belonging to one or other type, which is for example contained in aspecific octet T of the virtual subcontainer in question. Thisinformation, which is binary, has the advantage of being simple toprotect against any errors.

The octet T has its first seven bits expressing the binary word 1010101when the subcontainer is transporting useful data DU and the binary word0101010 when it is transporting padding data B. Thus, at the receivingequipment, the octet will be recognised by calculating the Hammingdistance between the first seven bits of the octet received and thevalue 1010101. If this distance is less than four, this means that theoctet T has the value 1010101 and the container received is transportinguseful data DU. On the other hand, if this distance is greater than orequal to four, this means that the octet T has the value 0101010 andthat the container received is transporting padding data B.

As before, the useful load CU will have a maximum length of 33 or 105octets and the maximum transmission D_(VC2) provided by a VC2 virtualcontainer (four S-VC2 virtual subcontainers) will then be per multiframe6720 kbits/sec and the rate D_(VC12) for a VC12 virtual container (fourS-VC12 virtual subcontainers) 2112 kbits/sec.

It will be understood that then, if N is the number of reservedcontainers on the SDH link, the total reserved rate will be N×2112kbits/sec for VC12 virtual containers and N×6720 kbits/sec for VC2virtual containers.

The unit 22 is controlled by the unit 23, which analyses the overheadoctets H (V5, J2, N2, K4), but also the octets L or T. Only the blocksof useful data DU are then delivered to the recovery unit 24.

Where padding data are inserted at the ATM physical layer rather than atthe containers, from the point of view of the SDH layer, all the datatransported are useful data without distinction.

The ATM layer supports this type of padding data. This is because theATM cells are structured as a header of five octets and a useful load in48 octets. It should be noted that, in AAL1, the useful load is reducedto 47 octets. The header makes it possible to define the type of loadtransported in the cell. The following value stored in the first fouroctets of the header indicates that it is a padding cell:

Octet 1 Octet 2 Octet 3 Octet 4 0000 0000 0000 0000 0000 0000 0000 0001

In order best to adjust the rate of the incoming flow to that of the Ncontainers reserved or concatenated in the SDH frame, the number N ofcontainers being fixed during the learning phase, the automatic ratecontroller of the ATM layer inserts, in each container, either paddingcells or useful data cells.

The size of a virtual container is 140 (4×35) or 428 (4×107) octetsaccording to the type of container processed by the equipment, VC12-xcor VC2-xc, including four overhead octets POH (V5, J2, N2 or K4), thatis to say in total 136 or 424 useful load octets, which will be used fortransporting the ATM cells which, for their part, contain 53 octets. Noalignment between the container and the ATM cells is effected so thatthe ATM cells can be distributed over two consecutive containers.

It will be understood that a VC12 virtual container transports 4×34octets of useful data per multiframe, which corresponds to a rateD_(vc12) of 2176 kbits/sec per C12 container. As for a VC2 virtualcontainer, this transports 4×106 octets of useful data per multiframe,which corresponds to a rate D_(VC2) of 6784 kbits/sec per C2 container.

In the case of the processing of the padding by direct introduction ofthe padding data, the automatic rate control process implemented by theunit is as follows.

If the difference between the values respectively carried by the highpointer ph and the low pointer pb of the buffer 12 is greater than thesize of one subcontainer, that is to say 33 or 105 octets, the automaticcontrol unit 16 extracts from the buffer 12 the useful data DU necessaryfor filling one or more containers, until the difference between thevalues ph and pb carried by the said pointers becomes less than the sizeof one subcontainer.

If this difference is less than the size of one subcontainer, then theautomatic control unit 16 delivers padding data B intended to beinserted in subcontainers, as previously described, until the saiddifference once again becomes greater than the size of one subcontainer.

A process implemented by the automatic rate control unit 16 is nowdescribed in the case of the processing of the padding at the ATM layer.If the difference between the respective values carried by the twopointers ph and pb of the buffer 12 is greater than the size of an ATMcell, that is to say 53 octets (or 47 octets in the case of AAL1), theautomatic rate control unit 16 extracts one or more data cells from thebuffer 12 until the difference between the two pointer values ph and pbare less than the size of one cell. If the difference between the valuesrespectively carried by the two pointers ph and pb of the buffer 12 isless than the size of one cell, then the automatic controller 16 insertspadding cells.

Where the padding B is inserted at the ATM physical layer, the paddingcells are rejected by the unit 22 in order to have only ATM cells anduseful data at the output 25.

1. Equipment for enabling the transmission of data by a synchronousnetwork, said equipment comprising at least one item of transmittingequipment and at least one item of receiving equipment, saidtransmitting equipment being arranged to respond to the data forinserting the data in concatenated virtual containers and for insertingthe virtual containers in synchronous frames for transmission to saidreceiving equipment, said transmitting equipment including a unit formeasuring the transmission rate of incoming data flow and aparameterizing unit for deriving, from the measurement made by saidmeasuring unit an indication of the total number of virtual containersof lower order to be virtually concatenated in order to transport saiddata flow in said frames, a header of each concatenated container beingarranged for carrying a message including indications of the totalnumber of concatenated containers and the order number of said containeramongst said concatenated containers.
 2. The equipment according toclaim 1 wherein said transmitting equipment is arranged to functionaccording to at least two distinct phases, an initialization phaseduring which the measuring unit is arranged for measuring the rate ofthe incoming flow and the parameterizing unit is arranged for derivingtherefrom an indication of the total number of virtual containers to bereserved in the synchronous frame in order to concatenate the virtualcontainers so as to transport said data of the incoming flow, followedby a normal operating phase during which the transmitting equipment isarranged for inserting said data in said reserved concatenated virtualcontainers, said transmitting equipment being arranged to exit from saidnormal operating phase, and to enter said an initialization phase inresponse to said measuring unit indicating that said measured rate isgreater than the maximum rate which can be offered by the total numberof reserved concatenated containers.
 3. The equipment according to claim1 wherein said transmitting equipment comprises a container formationunit for forming said concatenated containers in response on the onehand of the useful data of the incoming flow and on the other hand, ifthese containers are not available in sufficient quantity at the timethe concatenated containers are formed, by padding data necessary forsaturating the rate offered by said virtual containers.
 4. The equipmentaccording to claim 3, wherein each subcontainer has, in addition to itsheader, a length octet for representing the quantity of useful dataand/or the quantity of padding in its useful load.
 5. The equipmentaccording to claim 3, wherein said transmitting equipment has a bufferwhich is arranged to be supplied by said incoming flow and which isarranged to deliver blocks of useful data to said formation unit at itsrequest, and a control unit for selectively controlling (a) delivery bysaid buffer of one or more blocks of useful data when these data areavailable in said buffer and (b) delivering blocks of padding data whenthe quantity of useful data in said buffer is less than the quantity ofdata in a block.
 6. The data transmission system according to claim 5,wherein each of said blocks of data has the size of a virtualsubcontainer so as to fill the useful load of the virtual subcontainer,each virtual subcontainer being either of a type enabling it to containuseful data or of a type enabling it to contain padding data.
 7. Theequipment according to claim 6, wherein each subcontainer comprises, inaddition to its header, an octet representing the type of data which thesubcontainer contains.
 8. The equipment according to claim 7, whereinsaid network is of the SDH type in accordance with Recommendation G.707,and for a given concatenated virtual container, the transmittingequipment is arranged for causing said message to include sixteen J2header octets successively transmitted in sixteen successivemultiframes.
 9. The equipment according to claim 5 wherein said bufferis arranged for delivering the respective values of a high pointer and alow pointer, said control unit being arranged for delivering blocks ofpadding data when the difference between these two values is less thanthe size of a subcontainer, until said difference once again becomesgreater than the size of a subcontainer, said control unit then beingarranged for controlling the delivery, by the buffer, of the blocks ofdata which the buffer contains.
 10. Data transmission system accordingto claim 5, wherein said incoming flow includes a flow of ATM cells, andeach of said blocks of data has the size of one or more ATM cells. 11.Data transmission system according to claim 1 wherein said receivingequipment includes, at its output, a buffer which is arranged to (a) besupplied by the recovered data extracted from said concatenated virtualcontainers received, at the rhythm of the extraction of said data and(b) read the extracting data at a regular rhythm.
 12. The equipmentaccording to claim 1 wherein said message also carries an identifier forsaid transmitting equipment.
 13. A system including the transmitterequipment of claim 1 and a receiver adapted to be responsive to themessage transmitted by the transmitter equipment, said receivingequipment being arranged to deliver to an output thereof only thecontainers carrying said message, in the order given by said message.14. Data transmission system according to claim 13, wherein saidreceiving equipment is arranged to function according to at least twodistinct phases, a learning phase in which, by using the message carriedby each of said containers, said receiving equipment is arranged to markin the received frames an indication of the position of eachconcatenated container received, and a normal operating phase in which,on the basis of each position thus determined, the receiving equipmentis arranged to deliver said concatenated containers to a unit forrecovering said data.
 15. The equipment according to claim 13, whereinsaid transmitting equipment comprises a container formation unit forforming said concatenated containers in response on the one hand of theuseful data of the incoming flow and on the other hand, if thesecontainers are not available in sufficient quantity at the time theconcatenated containers are formed, by padding data necessary forsaturating the rate offered by said virtual containers; saidtransmitting equipment has a buffer which is arranged to be supplied bysaid incoming flow and which is arranged to deliver blocks of usefuldata to said formation unit at its request, and a control unit forselectively controlling (a) delivery by said buffer of one or moreblocks of useful data when these data are available in said buffer and(b) delivering blocks of padding data when the quantity of useful datain said buffer is less than the quantity of data in a block; each ofsaid blocks of data has the size of a virtual subcontainer so as to fillthe useful load of the virtual subcontainer, each virtual subcontainerbeing either of a type enabling it to contain useful data or of a typeenabling it to contain padding data; and said type octet has seven bitswhich express the word 1010101 when said subcontainer is transportinguseful data and the word 0101010 when it is transporting padding data,the receiving equipment being arranged for (a) recognizing said octet bycalculating the Hamming distance between the seven bits of the octetreceived and the value 1010101 and by comparing this distance with thenumerical value four, (b) receiving a subcontainer including useful dataif the distance is less than four and (c) receiving a subcontainerincluding padding data if the distance is greater than or equal to four.16. The system according to claim 13, wherein said transmittingequipment comprises a container formation unit for forming saidconcatenated containers in response on the one hand of the useful dataof the incoming flow and on the other hand, if these containers are notavailable in sufficient quantity at the time the concatenated containersare formed, by padding data necessary for saturating the rate offered bysaid virtual containers; each subcontainer has, in addition to itsheader, a length octet for representing the quantity of useful dataand/or the quantity of padding in its useful load; and said receivingequipment is arranged for storing blocks of said data in a multiframe byconsecutively filling the reserved containers.
 17. The system accordingto claim 13 wherein said receiving equipment is arranged for storingblocks of the data in a multiframe by distributing, one after the other,said blocks on the reserved containers.
 18. A method of supplying datafor transmission by a synchronous network to a receiver, said data,prior to transmission, being inserted in virtual containers which arethemselves inserted in synchronous frames in order to be transmitted,wherein the method comprises measuring the rate of the flow of incomingdata to be transmitted, deriving from said measurement an indication ofthe total number of virtual containers to be concatenated in order totransport said data flow in said frames, and providing each of saidcontainers to be concatenated with a message indicating the total numberof concatenated containers as well as the order number of said containeramongst said concatenated containers.
 19. Data transmission methodaccording to claim 18, wherein said network is of the SDH type inaccordance with Recommendation G.707 and for a given concatenatedvirtual container, said message includes sixteen J2 header octetssuccessively transmitted in sixteen successive multiframes.
 20. The datatransmission method according to claim 18 in combination with receivinga frame including the concatenated containers, performing two distinctsteps on the second frame, the first step being a learning step inwhich, in response to the message carried by each of said containers,the position of each concatenated container received is recovered in theframes received, the second step being a normal functioning step inwhich, on the basis of each position thus determined, said concatenatedcontainers are delivered so that said data can be recovered.
 21. Datatransmission method according to claim 20 further including putting backin phase, on reception, the concatenated containers out of phase byseveral multiframes by analyzing the phase difference between octetsconstituting the message carried by each of them.
 22. Data transmissionmethod according to claim 18 wherein, on transmission, two distinctphases are performed: an initialization phase during which a measurementof the incoming flow rate is carried out and an indication of the totalnumber of virtual containers to be reserved for the transportation ofsaid data of the incoming flow is derived, and a normal functioningphase during which said data of the incoming flow are inserted in saidreserved virtual containers, said normal functioning phase beingreplaced by said initialization phase when said measured rate is greaterthan the maximum rate which can be offered by the total number ofreserved concatenated containers.
 23. Data transmission method accordingto claim 18 further comprising using, for forming said concatenatedcontainers, on the one hand useful data of the incoming flow and on theother hand, if these useful data are not available in sufficientquantity at the time the concatenated containers are formed, usingpadding data necessary for saturating the flow offered by said virtualcontainers.
 24. Data transmission method according to claim 18 whereinsaid message also carries an identifier for said transmitting equipment.25. A method of forming a synchronous frame for the transportation ofdata flows, comprising inserting said data in virtual containers thatare themselves inserted in said synchronous frame, concatenating saidcontainers with each other, and causing each concatenated container tocarry a message giving an indication of the total number of concatenatedcontainers as well as the order number of said container amongst saidconcatenated containers.
 26. The method according to claim 25, whereinthe frame is of the SDH type in accordance with Recommendation G.707,and for a given concatenated virtual container, said message includessixteen J2 header octets successively transmitted in sixteen successivemultiframes.
 27. The method according to claim 25 further includinginserting into each container, a header, a type octet which indicateswhether the container is carrying, in its useful load, useful data orpadding data.
 28. The method according to claim 27, wherein said typeoctet has seven bits which express the word 1010101 when saidsubcontainer is transporting useful data and the word 0101010 when it istransporting padding data.
 29. The method according to claim 25, furtherincluding inserting into each container, a header, a length octet whichrepresents the quantity of useful data and/or the quantity of padding inits useful load.